There has been known a technique in which an NOx storage reduction catalyst (hereinafter referred to simply as a NOx catalyst) is arranged in an exhaust passage of an internal combustion engine. The NOx catalyst has a function to occlude NOx in an exhaust gas when the oxygen concentration of the exhaust gas flowing into the NOx catalyst is high, and to reduce the NOx occluded in the NOx catalyst when the oxygen concentration of the incoming exhaust gas decreases and when there exists a reducing agent.
In addition, a sulfur component contained in fuel is also occluded to the NOx catalyst, similar to NOx. The sulfur component occluded in this manner is more difficult to be released than NOx, and accumulated in the NOx catalyst. This is called sulfur poisoning. The NOx purification rate of the NOx catalyst is decreased by this sulfur poisoning, so it is necessary to perform a sulfur poisoning recovery process for recovering the NOx catalyst from the sulfur poisoning at an appropriate time. This sulfur poisoning recovery process is performed by making the temperature of the NOx catalyst high and passing an exhaust gas of a stoichiometric air fuel ratio or a rich air fuel ratio to the NOx catalyst. For example, by adding fuel to the NOx catalyst, the fuel reacts with the NOx catalyst whereby the NOx catalyst is made high in temperature. By further adding fuel to the NOx catalyst under such a condition to control the air fuel ratio of the exhaust gas to a rich air fuel ratio, the sulfur poisoning can be recovered.
Also, there has been known a technique in which a particulate filter (hereinafter referred to simply as a filter), which carries a catalyst with oxidation ability or has a catalyst with oxidation ability arranged at an upstream side, is provided for collecting particulate matter (hereinafter referred to as PM) in an exhaust gas. When the amount of PM collected in the filter reaches a predetermined amount, a reducing agent is supplied to the catalyst having oxidation ability, so that the temperature of the filter is raised to remove the PM therein through oxidation. To remove the PM in the filter in this manner is called the regeneration of the filter. Hereinafter, the sulfur poisoning recovery process is called S regeneration, and to remove the PM through the oxidation thereof is called PM regeneration.
Further, there has also been known a technique that is provided with an HC addition valve, an oxidation catalyst, a particulate filter, an HC addition valve, an NOx storage reduction catalyst, and an NOx selective reduction catalyst on an exhaust passage in order from an upstream side thereof to a downstream side thereof (see, for example, a first patent document).
First Patent Document: Published Japanese Translation 2006-512529